In the automatic toll collection industry, it is necessary to accurately identify each vehicle passing along the roadway in order to determine the number and amount of tolls due. Without a human toll collector, the automatic machinery must be able to discriminate and to categorize the different types of vehicles. For instance, it must be able to categorize a vehicle as a car or a truck, or as a truck of a particular size, if different tolls are to be levied.
Another problem is discriminating a car pulling a trailer on a trailer hitch, which may be detected as two separate vehicles requiring two tolls, while a first car closely tailgating a second car might be sensed as a single vehicle. Conventional loop detectors, which detect the presence of large metallic objects (i.e. cars and trucks) from a changing magnetic field, have proved less than completely satisfactory in distinguishing between these two situations.
While optical vehicle detection devices have been employed, they are prone to problems from reflected sunlight which interferes with a scanning light beam. An example of such an optical detector is shown in U.S. Pat. No. 2,697,948 (Pratt) in which a stationary fluorescent light directs a light beam across a roadway to a phototube and cooperates with a treadle positioned in the ground across the roadway to detect and classify vehicles. This patent recognizes that under its own construction it is necessary to have a source of light of unvarying intensity. While the Pratt system compensates for fog or haze by ignoring a gradual light intensity change, there remains the problem of clouds moving across the sun and creating rapid changes in the intensity of light received by the scanning device.
Furthermore, in discriminating and categorizing vehicles, it is sometimes necessary to identify even relatively small objects which intercept the scanning beam. An example of such a small object is a trailer hitch, which must be accurately detected to discriminate between the car/trailer combination and the two car tailgating situation mentioned above. The scanning beam must be adapted for precise detection over a range of small to large objects so as to adequately discriminate between these two situations.